Conventional permanent magnet type synchronous motors in which some portions of an iron core are protruded among the poles of permanent magnets of the rotor are disclosed, for instance, in FIGS. 1(a), 1(b) and 1(c) of Japanese Patent Laid-Open No. 62-155796 (1987) published on July 10, 1987 entitled "Height Power Factor Control Method of Permanent Magnet Type Synchronous Motor" and FIGS. 1 to 4 of Japanese Utility Model Laid-Open No. 62-88463 published on June 5, 1987 entitled "Rotor of Permanent Magnet Type Synchronous Motor". These publications discuss an optimum ratio of the direct axis inductance of the rotor to the quadrature axis inductance thereof to be set at the time of positive utilization of reluctance torque in a current controlling 180.degree. electric PWM control system in which the lead and lag of current phase are controlled in accordance with the magnitude of an amperage detected by a current sensor provided therein so as to detect the electric current flowing to the motor.
The circuit of a motor thus constructed and applied to an industrial sewing machine is shown in FIG. 1. FIG. 1 shows a permanent magnet type synchronous motor 1 (which will hereinafter be referred to as motor), magnetic pole sensor 2 adapted to detect the magnetic pole position of the motor 1, encoder 3 adapted to detect the rotational speed and direction of the motor 1, driving circuit 4 for the motor 1, which normally works as an inverter driving circuit and is driven by a normal rotation instruction 13 or a reverse rotation instruction 14 from a speed-position control circuit 5, operational speed instruction circuit 6, sewing machine 7 constituting a load, needle position sensor 8 for the sewing machine 7 which is adapted to normally detect two positions, i.e. the lower and upper positions of the needle, a signal processing circuit 9 adapted to determine the rotational speed (revolution numbers) in the normal rotational and reverse rotational directions on the basis of a detected original signal from the encoder 3 and output a signal to the speed-position control circuit 5, and a sewing machine control circuit 10 associated with the function of the sewing machine and adapted to drive the sewing machine 7 in accordance with a signal from a sewing machine operating instruction circuit 11 which is energized by a sewing command signal from an operator. The motor 1 and sewing machine 7 are connected together by a belt, and this motor 1 is of a 120.degree. feed PWM voltage control system. Reference numeral 12 denotes a belt connecting the motor 1 and sewing machine 7 together.
When a sewing command from the operator is sent to the sewing machine operating instruction circuit 11 with an operational speed command to the operational speed instruction circuit 6, a signal is sent from the sewing machine control circuit 10 to the speed-position control circuit 5 so as to operate the motor 1 in accordance with the speed command. The speed-position control circuit 5 selects at the acceleration time a normal rotational mode on the basis of an operation command signal to accelerate the motor 1 by the driving circuit 4 at a voltage command level which is 120.degree. feed pulse width controlled, and enter into a steady operation. During this time, a stator winding to which an electric current is to be selectively applied is determined by processing a signal from the magnetic pole sensor 2 by the speed-position control circuit 5, and a plurality of selected transistors in the inverter are turned on in accordance with a signal from this circuit 5, so that a winding current flows.
A signal obtained from the encoder 3 and representative of an actual speed of the motor 1 is fe back to the speed-position control circuit 5 and sewing machine control circuit 10, and the operational speed is in agreement with this signal and a speed command applied from the operational speed instruction circuit 6 to the control circuit 5 by a foot pedal (not shown).
After a predetermined sewing step in a sewing machine has been finished, to stop the operation of the sewing machine, an instruction is given out from the operation instruction circuit 11, and a reverse instruction 14 is applied from the position-speed control circuit 5 to the inverter driving circuit 4 to start the inversion deceleration (generate inverting torque). As a result, the speed of the motor 1 decreases. When the speed of the motor has reached a level at which the motor can be stopped, the needle position sensor 8 mounted on the sewing machine 7 detects a stop position which is previously decided in the sewing machine 7, the upper portion or lower portion, which is determined by the sewing machine operating instruction circuit 11, of the needle of the sewing machine 7 is selected, and the inversion braking force is cut off to stop the sewing machine.
A machine operated in this manner must carry out several-switch sewing operations with a high frequency.
A motor used in a sewing machine is thus started and stopped very often, so that a large current flows therethrough every time the motor is started and stopped. Industrial sewing machines and Factory Automation Robots are required to start and stop high frequently. Since large torque is required and feed current becomes large in this type sewing machine at the time of acceleration or deceleration of the motor as disclosed in FIG. 2, the temperature of the motor 1, the driving circuit 4 and other constituting elements rise remarkably during the operation of these elements so that these elements are required to have large capacity for avoiding the temperature rising of these elements.
The above-mentioned prior publications do not discuss a permanent magnet type synchronous motor of a 120.degree. electric PWM voltage control system in which the control of the lead and lag of current phase is done. At the same time, these publications do not discuss a control method of a motor which carries out acceleration or deceleration frequently, namely the motor, carries out normal rotation to reverse rotation very frequently.
FIG. 3 shows torque characteristics of a rotational phase of a permanent magnet type synchronous motor of a 120.degree. electric PWM voltage control system which has protruded iron cores between the poles of permanent magnets of the rotor. In FIG. 3, the solid line shows a torque characteristic of a conventional permanent magnet type synchronous motor which does not have the protruded iron cores among the poles of permanent magnets of the rotor, and the dotted line shows a permanent magnet type synchronous motor of the present invention which has protruded iron cores (projecting poles) therebetween. Referring to FIG. 3, the permanent magnet type synchronous motor of the present invention having the projecting poles generates a large torque compared with the conventional permanent magnet type synchronous motor which lacks the projecting poles under the same applied current. A motor torque (.sym. torque) in the normal rotation region or in a state of braking in which the motor is changed from the reverse rotation region to the normal rotation region, is delayed in phase by .alpha..degree. compared with that of the conventional motor as can be seen in the large torque generation portion from (30.degree.+.alpha..degree.) to (30.degree.+.alpha..degree.)+(30.degree.) in FIG. 3. Another motor torque (.crclbar. torque) in the reverse rotation region or in another state of braking in which the motor is changed from the reverse rotation region to the normal rotation region, is advanced by a phase .beta. compared with that of the conventional motor as disclosed in another large torque generation portion from {(180.degree.)-(30.degree.-.beta..degree.)-(30.degree.)} to {(180.degree.-(30.degree.-.beta..degree.)} in FIG. 3. As explained above, the conventional permanent magnet type synchronous motor which lacks the projecting poles has a drawback in that a large torque can not be attained. However, the above-mentioned prior publications do not discuss this point.
In the synchronous motors disclosed in the above-mentioned prior publications, consideration is not given to a temperature rise in the constituent parts and a lack of the resistance thereof to an increase in the current capacity, which would cause troubles when the accelerating and decelerating of the engine is done with frequency.
The construction of a rotor in a permanent magnet type synchronous motor to which the present invention is applied will now be described with reference to FIG. 4. Referring to the drawing, reference numeral 101 denotes a rotor as a whole, 102 a permanent magnet fixed to the outer circumferential portion of a rotor core 103, and 104 a detecting magnet adapted to determine a commutating period of a stator winding (not shown) and fixed to a motor shaft 105. As is clear from a sectional view along A--A line of FIG. 4, which is shown in FIG. 5, the permanent magnet 102 is divided into four equal parts which are bonded to the outer circumferential surface of the rotor core 103 with a bonding agent. The permanent magnets 102 may also be held on the outer circumferential surface of the rotor core 103 by a small-thickness cylindrical member of stainless steel. If both the bonding agent and cylindrical member are used, the permanent magnets can be fixed to the rotor core more firmly.
Referring to FIG. 4 the prior art detecting magnets 104 comprising a metal magnetized body are arranged in same angle and same polarity as the permanent magnets 102 facing to the side surface of the permanent magnets 102. If three phases are used for the stator coils, three magnetic pole detecting elements 30 are necessary on the magnetic pole sensor 2 as disclosed in FIG. 6.
A rotor in a conventional motor of this kind is constructed as shown in FIG. 4.
According to the construction of the rotor shown in FIG. 4, a large increase in reactance torque cannot be expected due to the reactance. The synchronous motor in which this problem is solved that is to increase the torque, are the synchronous motors disclosed in the previously-mentioned laid-open publications. However, even in these synchronous motors, for example, the reduction of cogging torque is not considered.